(a) Field of the Invention
This invention relates to a field emission cold cathode for use in an electron gun for a cathode ray tube (hereinafter referred to as a "CRT") and a monitor display unit having a high luminance and a high resolution on a screen.
(b) Description of the Related Art
A thermionic (hot) cathode has been generally used as a conventional electron source for an electron gun in a CRT. Today, a monitor display unit for computers having a high luminance and a high resolution is demanded, which requests the electron gun to operate under a current density as high as the critical density of the thermionic cathode.
On the other hand, although electronic components are generally requested to consume less electric power and to be less environmentally hazardous, it is difficult for an electron source having the thermionic cathode to satisfy the request for the lower electric power. Therefore, a new type of electron source capable of satisfying the request is sought for.
A new type of electron gun for a CRT, which employs a field emission cold cathode, has been proposed in JP-A-7(1995)-21903, for example. FIG. 1A is a cross-sectional view of the proposed field emission cold cathode, and FIG. 1B is a schematic top plan view an showing the relative location between a cathode area and a feeder area as viewed in the direction perpendicular to the layers in the field emission cold cathode.
An insulating zone 29 implemented by a first insulation layer or field oxide film extending along an overlying cathode area 34 is formed on a silicon substrate 27. The insulating zone 29 has a substantially circular outer periphery apart radially outside by distance L from the outer periphery of the cathode area 34. A resistance layer 30, which is electrically connected with the silicon substrate 27 through a feeder area 28 of an annular substrate area, is formed on the entire surface including the surfaces of the insulating zone 29 and the feeder area 28. A second insulation layer 31 and a gate electrode layer 32 are formed on the resistance layer 30. A multiplicity of substantially cylindrical holes are formed in a circular cathode area 34 overlying the insulating zone 29 from the surface of the gate electrode layer 32 to the bottom of the second insulation layer 31 to expose the surface of the resistance layer 30. A minute conical emitter 33 is disposed in each of the cylindrical holes for emitting electrons.
In operation, when the tips of the conical emitters 33 are subjected to an electric field of about 10.sup.8 V/cm generated by a voltage applied between the silicon substrate 27 and the gate electrode layer 32, electrons are emitted from the tips of the conical emitters 33 by a tunnel effect. Where the diameter of the cylindrical holes and the thickness of the second insulation layer 31 are both on the order of 1 .mu.m, the electric field obtained in the vicinity of the tips of the conical emitters 33 is on the order of several tens of volts at most.
The silicon substrate 27 and the gate electrode layer 32 function as a parallel plate capacitor for storing electric charge therebetween. The accumulated electric charge may often cause an instantaneous discharge to generate a temporary short-circuit between the emitters 33 and the gate electrode layer 32 due to local deterioration of vacuum or other reason. In this case, the temporary short-circuit may generate a destructively high temperature beyond the melting point of the emitters 33. The resistance layer 30 is provided for the purpose of absorbing the excessive instantaneous current caused by the temporary short-circuit to thereby protect the emitters 33 from a thermal destruction.
The distance L between the feeder area 28 and the cathode area 34 as viewed in the direction perpendicular to the layers is provided to increase the resistance in this part of the resistance layer 30 to lower the voltage drop across the portion of the resistance layer 30 disposed within the span of the cathode area 34.
In the field emission cold cathode as described above, a current density as high as 100 to 1000 A/cm.sup.2 can be attained for an emitter density of 10.sup.8 emitters/cm.sup.2, which is 10 to 100 times as high as that of the thermionic cathode. Since electrons are emitted by the tunnel effect in the field emission cold cathode, no heater is needed and accordingly power consumption can be saved. Thus, a monitor display unit having a high luminance and a high resolution with a low electric power consumption is realized for computers by taking these advantages of the field emission cold cathode.
In a field emission cold cathode having a larger cathode area 34, however, there is a tendency in which the electric potential at the conical emitters 33 is higher as they are located nearer to the center of the cathode area 34 or more distant from the feeder area 28. Accordingly, the current which can be taken out of the conical emitters in the vicinity of the center of the cathode area is smaller to degrade the current density.
FIG. 2 shows a calculated current distribution curve within the cathode area 34 shown in FIGS. 1A and 1B. The axis of abscissa shows the distance from the center of the cathode area 34, and the axis of ordinates shows the current in an arbitrary unit. As understood from FIG. 2, the most part of the current is provided from the emitters located near the feeder area 28 or in the vicinity of the outer periphery, whereas the emitters located in the vicinity of the central part of the cathode area 34 contribute little to the emission.
FIG. 3 shows calculated currents against voltages applied between the emitter 33 and the gate electrode layer 32 for two cases: one where the resistance layer 30 is provided as shown in FIGS. 1A and 1B; and the other where the resistance layer is omitted. As understood from FIG. 3, there is a tendency in which the current difference between the two cases becomes larger as the emitter current increases as a whole. The characteristic of the field emission cold cathode shown above requests a higher driving voltage of the cathode, renders the driving circuit complicated, and increases the electric power consumption.
Some measures for improving the above-mentioned unevenness of current within the cathode area 34 are proposed, as in JP-A-7(1995)-153369 and-JP-A-7-282716, for example. FIG. 4 is a cross-sectional view of the field emission cold cathode proposed by the former publication, wherein there are provided an annular cathode line 19 formed on an insulator substrate made of glass, for example, and a plurality of cathode conductor islands 20 formed separately from the annular cathode line 19 within an area encircled by the cathode line 19. The cathode line 19 and the cathode conductor islands 20 are electrically connected through a resistance layer 21 formed on the cathode line 19 and the cathode conductor islands 20. It is recited that the emissions from the conical emitters disposed within the span of the cathode conductor islands 20 as viewed in the direction perpendicular to the layers are uniformalized due to an approximately constant resistance between the conical emitters and the cathode conductor islands 20.
In the field emission field cathode as described above, however, electric charge accumulated between the cathode conductor islands 20 and the gate electrode layer 23 may often be released due to a temporary short-circuit between the conical emitters and the gate electrode layer 23. In such a case, a large discharge current flows along the vertical direction of the resistance layer 21 having a limited resistance in the vertical direction, and causes an excessive current to flow through the conical emitters and destroys them. The destruction often results in a permanent short-circuit failure between the conical emitters and the gate electrode layer 23, causing a fatal defect in the CRT.
In a field emission display device (FED) or a liquid crystal display device (LCD), it is proposed, in JP-A-7-32632 (for FED) and JP-A-7-104244 (for LCD), for example, to provide a plurality of terminals in the scanning block for avoiding voltage drops in the supply lines. Referring to FIG. 5 illustrating the LCD structure shown in the latter publication, gate signal lines each opposed to a common electrode 24, with an intervention of a LCD plate therebetween, extend along the horizontal direction of a display screen as scanning metallic lines. A plurality of terminals are provided for the common electrode 24 to thereby receive separately adjusted voltages. Specifically, the voltages separately adjusted by a plurality of electric sources 25 and 26 are supplied to the respective terminals to form a voltage slope in the common electrode 24. Thus, if uneven voltages are applied by the switching devices disposed along the gate signal lines, the uneven voltages and hence unevenness of luminance in the display screen are compensated by the configuration of the plurality of terminals. Voltage differences in the scanning lines near and remote from the voltage source occur due to the voltage drops occurring in the signal lines, although the voltage drops might be desired to be reduced to zero and are in fact unavoidable in its nature.
Referring back to FIGS. 1A and 1B, the slope of voltages occurring within the cathode area 34 shown in these figures is caused by the resistance layer 30 extending underneath the conical emitters 33. However, the resistance layer 30 is provided for the purpose of suppressing an excessive current flowing in the event of temporary short-circuit between the emitters 33 and the gate electrode layer 32. Therefore, it is unreasonable to eliminate the resistance layer 30 in a field emission cold cathode, different from the above-described examples for a LCD in which the resistance in the signal line may be desired to zero.